1. Field of the Invention.
The present invention relates to electric submergible pumping systems for recovering liquids and gas from a wellbore and, more particularly, to submergible pumping systems adapted to recover a higher volume percent of gas than previous pumping systems.
2. Description of Related Art.
Electric submergible pumping systems are commonly used to recover liquids from subterranean wellbores, and generally comprise an electric motor that operates a multistage centrifugal pump. Conventional centrifugal pumps tend to become gas locked and cease moving fluids when the gas content in the liquids exceeds a certain volume percent, which depends upon the gas pressure and the type and size of pump. In many oil and gas reservoirs the gas content of the fluids recovered to the surface can exceed the gas moving ability of centrifugal pumps. There is a need for an improved electric submergible pumping system to recover liquids and gas.
Those skilled in the art believe part of the problem of centrifugal pumps being able to recover liquids with relatively high gas content is caused by the configurations of the impellers. Centrifugal pumps are formed from a plurality of pump stages, with each stage including a stationary diffuser and a rotating impeller. A typical impeller is formed from an impeller body having an upper surface spaced from a lower surface with a plurality of angled vanes therebetween to define a plurality of flow chambers. A balance hole extends through the upper surface into each of a plurality of the flow chambers. In a typical centrifugal pump impeller, shown in FIG. 3, the fluid pressure in the area labeled "A" (Pa) is always less than the fluid pressure in the area labeled "B" (Pb) due to the centrifugal force generated by the impeller. If balance holes (numbered as "50" in FIG. 3) were not present, then the fluid pressure in area "C" (Pc) would be approximately equal to Pb. Additional downthrust would then be generated that directly leads to pump bearing failure. If Pc is decreased to be approximately equal to Pa, by the addition of the balance holes ("50"), the detrimental downthrust will be decreased. If Pa and Pc become approximately equal, then the pressure difference between Pb and Pc will be too great. In order to keep the fluid pressure in area "C" (Pc) at a desired level, fluid leakage between areas "B" and "C" should be minimized. Additionally, fluid leakage from area "D" to "A" and from area "B" to area "D" needs to be reduced to decrease the pressure drop across area "B".
In typical centrifugal pumps, as the volume percentage of gas in the fluid being moved increases a gas pocket or bubble will form in area "A". If the gas content becomes too great a relatively large gas bubble (shown in dotted lines) will form and effectively block all fluid flow therepast. In this case, the pump will cease to move fluid, and the pump will be referred to as being "gas locked". Additional equipment, such as rotary gas separators, are sometimes used on the inlet side of the centrifugal pump to remove as much gas as possible prior to the fluids being introduced thereinto. These gas separators have proven to be generally effective; however, the addition of a gas separator involves additional monies that must be spent on a well and adds additional components that can fail. In addition, in certain enhanced oil recovery projects, such as water, gas or polymer floods, the volume percentage of gas in the recovered fluids can vary greatly over the life of the project. Therefore, the well may not need a gas separator but for a relatively short period of time.
There is a need for an improved electric submergible pumping system that can move liquids with a relatively high gas content without experiencing gas locking, and which does not necessarily require the use of a gas separator.